1. Field of the Invention
This invention relates generally to tracking devices, and more particularly to estimating the azimuth pointing angle of a monopulse antenna.
2. Description of the Related Art
Currently, monopulse detection systems are widely utilized in airborne and spaceborne radar systems to locate moving targets. A monopulse detection system detects an azimuth angle to a target and thereafter locates the absolute position of the target based on the monopulse antenna azimuth and elevation azimuth pointing angles and the relative azimuth angle to the target. In particular, a signal is radiated from a transmitting antenna and reflected off the target. The reflected signal then is received at the monopulse detection system through two or more receiving feeds. Utilizing the phase between the signals received at the individual feeds, data regarding the azimuth angle of the target to the detection system is determined, as illustrated in FIG. 1.
FIG. 1 is a schematic diagram showing an exemplary prior art monopulse detection system 100. The monopulse detection system 100 includes a signal generator 102 coupled to a transmitting feed 104. In addition, two receive channels 110 and 114 are located to either side of a feed axis boresight 118. The receive channels 110 and 114 each include a receiving feed 106 and 108 coupled to signal processing logic 116.
In operation, the signal generator 102 generates a signal, which is radiated or emitted from the transmitting feed 104. When a target is within the cross-range of the monopulse detection system 100, the target reflects the radiated signal, which in turn is received by two receiving feeds 106 and 108. The received signals are converted into low frequency signals and subjected to signal analytic processing utilizing the signal processing logic 116.
The signal processing logic 116 adds the signals received from the two receiving feeds 106 and 108 to obtain a sum pattern. In addition, the signal processing logic 116 calculates a difference between the signals received from the two receiving feeds 106 and 108 to obtain a difference pattern. That is, the signal received from one receive channel, such as receive channel 114, is subtracted from the signal received from the other receive channel, in this example receive channel 110. The resulting difference pattern has a property wherein the difference pattern is positive when the target is located on the side associated with receive channel 110. Then, as the target crosses the boresight 118 of the feed axis, the difference pattern becomes negative because the signal is stronger in the side associated with receive channel 114. Hence, the difference pattern is null in the middle, at the boresight 118, positive on one side of the boresight 118 and negative in the other side of boresight 118.
The sum pattern is used to indicate the target is in the cross-range of the monopulse detection system 100, while the difference pattern is used to determine where the target is relative to the boresight 118. Hence, in the example above, when the target is to the side associated with receive channel 110 the difference pattern is positive. When the target is to the side associated with receive channel 114 the difference pattern is negative. Finally, when the target is in the middle of the receive channels 110 and 114 there is a null in the difference pattern because the energy coming into the two feeds 106 and 108 is equal. Thus, when a strong signal is present in the sum pattern and the difference pattern is null, the target is located along the boresight 118 of the monopulse detection system 100.
As can be appreciated, the above described monopulse measurement provides an angle measurement relative to the boresight 118 of the monopulse detection system 100. To obtain an absolute measurement of the target's location in space, the monopulse antenna azimuth pointing angle should be known. The monopulse antenna azimuth pointing angle is the position angle of the boresight 118 of the monopulse detection system 100, referred to hereinafter as the monopulse null angle. For example, if the monopulse detection system 100 is a radar in orbit and a target is detected on the ground, a monopulse measurement will detect the target's location as an angle relative to the boresight 118 of the radar. However, to obtain the latitude and longitude of the target, the monopulse null angle of the radar should be determined.
In the prior art, the orbiting radar might, for example, include a housing holding a star tracker and gyroscopes. The star tracker and gyroscopes can then be utilized to determine the position and orientation of the housing, which is located a distance away from the antenna. To get absolute positional information regarding the monopulse null angle, the angle between the antenna boresight 118 and the star tracker housing must be determined with extreme accuracy, which is a very cumbersome operation to perform. Moreover, small inaccuracies in the monopulse null angle can introduce large errors in the monopulse measurement angle of a moving target relative to the boresight 118, particularly for spaceborne radar systems. These errors limit the accuracy with which the absolute position of the moving target can be established. As a result, the usefulness of spaceborne radar systems designed to detect and locate moving targets is limited.
In view of the foregoing, there is a need for systems and methods that accurately determine the monopulse null angle of a monopulse detection system. The systems and methods should be capable of measuring the monopulse null angle regardless of antenna misalignments or instabilities. Hence, the systems and methods should not rely on separate detection mechanisms such as star trackers or gyroscopes.